I think they have stretched the 737 to its limit...
It could be done though . . . off course the plane now needs some longer legs, in order to prevent tailstraikes at rotation. Furthermore, a larger wing would be desirable, since anymore lift out of the existing wing would not be possible. The larger wing would hold more fuel, giving it true transcon range [no payload restrictions], possibly trans atlantic. The larger wing, more fuel, and stretched fuselage give consedarble higher MTOW. Now we need some nice efficient engines [RR/PW]. Since were giving the 737-900 design a nice refurbishment, finishing touches are a new cockpit design to give some more speed... Oh wait...hell...it looks like a 757!!
Seriously, the -800 was stretched in order to give the -800 capacity some more legroom, and increase two class seating capacity. The -800/900 design is limited in capacity by the 90 second evacuation time. In order to increase capacity even further, additional [or larger] exits are required. So the limited -800 stretch to the -900 did not increase max. capacity, however it did increased average space per pax. This is why KLM purchased the -900.
Immigration officer: "What's the purpose of your visit to the USA?" Spotter: "Shooting airliners with my Canon!"